1) Field of the Invention
The present invention relates to a toner image transfer method, a toner image transfer device, and an image forming apparatus.
2) Description of the Related Art
An image forming apparatus forms an electrostatic latent image onto a latent image carrier, develops the electrostatic latent image to obtain a toner image, and transfers the toner image onto a paper and fixes the toner image, thereby to obtain an image. A digital copying machine, an optical printer, an optical plotter, and a facsimile machine or the like are the example of the image forming apparatus. Color image forming apparatuses capable of forming color images have also appeared on the market.
A one-drum type and a tandem type are the major types of the color image forming apparatuses.
The one-drum type color image forming apparatus has a single drum-shaped latent image carrier. The one-drum type color image forming apparatus forms and develops electrostatic one-color latent images of three or four colors on the latent image carrier. The colors are selected from magenta, yellow, cyan, and black. The one-color latent images are transferred and superimposed onto one paper thereby obtaining a full color image.
The tandem type color image forming apparatus has a drum-shaped latent image carrier for each of the three or four colors. An electrostatic latent image corresponding to a predetermined color is formed on a corresponding one of the latent image carrier. The latent images are then developed with a toner of corresponding color, thereby to obtain color toner images. The color toner images are then transferred and superimposed onto a paper thereby obtaining a full color image.
Two transferring methods are known for transferring the toner images onto the paper: direct transfer and intermediate transfer. In the direct transfer, color toner images are directly transferred from the latent image carriers to the paper. In the intermediate transfer, the toner images on the latent image carriers are first transferred to an intermediate transfer medium such as an intermediate transfer belt and then transferred onto the paper.
In both types, a toner image is transferred and superimposed on the toner image that is transferred earlier. However, many times the toner image transferred earlier is not completely fixed. If a toner image is superimposed over an earlier not-fixed toner image, toner of the not-fixed toner image gets adhered to the latent image carrier. This phenomenon is called reverse transfer.
The reverse transfer disturbs the toner image that is transferred earlier, and this becomes a cause of degrading the quality of the color image that is finally obtained.
One approach is to clean residual toner from the latent image carrier before transferring a new image. The residual toner may be collected and reused. However, the toner recovered contains a mixture of toners of different colors so that the toner recovered can not be used as it is, or if used, color reproducibility of a color image or a multi-color image is lost substantially.
As described in Japanese Patent Application Laid-open No. H9-146334, one approach to reduce the reverse transfer is to set a contact angle of the latent image carrier relative to water equal to or more than 85 degrees. However, this approach is insufficient to reduce the reverse transfer.
As described in Japanese Patent Application Laid-open No. H7-271201, another approach to reduce the reverse transfer is to run the intermediate transfer medium faster than the latent image carrier.
The inventor has also confirmed the effect of this method by experiment. FIG. 1 is a graph of a reverse transfer rate of a yellow toner image (at the right ordinate) and a transfer rate of a magenta toner image (at the left ordinate), when the running speed of the intermediate transfer medium (i.e., the intermediate transfer belt) is different from that of the latent image carrier (i.e., a drum-shaped photoconductive photosensitive member).
The abscissa of the graph shown in FIG. 1 represents a linear velocity ratio that is defined as (Vb−Va)/Va)×100 (%), where Va is the running speed of the latent image carrier, and Vb is the running speed of the intermediate transfer medium. The linear velocity ratio is zero when Vb is equal to Va, that is, when the running speed of the latent image carrier is equal to the running speed of the intermediate transfer medium.
When an absolute value of the linear velocity ratio becomes larger, a reverse transfer rate 1-1 of the yellow toner image decreases, and a reverse transfer rate 1-2 of the magenta toner image increases.
Thus, when the running speed of the latent image carrier is different from that of the intermediate transfer medium, the transfer rate improves and the reverse transfer rate decreases. This is considered for the following reason. When the running speeds are set different, a relative displacement occurs between the latent image carrier and the intermediate transfer medium. The toner image that is in a stable state on the latent image carrier becomes in an unstable state, and Van der Waals' forces between the toner image and the latent image carrier decrease. Electrostatic adhesive force to the latent image carrier effectively decreases when a distance between the toner and the latent image carrier increases. Therefore, the transfer rate increases, and the reverse transfer rate decreases.
However, if the running speed of the latent image carrier if different from that of the intermediate transfer medium, although the reverse transfer does not occur, the image quality lowers.
In other words, a transfer section where the toner image is transferred from the latent image carrier to the intermediate transfer medium is formed as a nip section where the latent image carrier and the intermediate transfer medium are brought into contact with each other. During a period when the toner image that is transferred onto the intermediate transfer medium passes through the nip width of the transfer section, the side of the toner image that is in contact with the intermediate transfer medium and the side of the toner image that is in contact with the latent image carrier receive mutually opposite forces in the running direction because of the difference in the running speeds.
Therefore, when the toner passes through the transfer section, the toner image is deformed to be extended to the running direction.
FIG. 2 is an explanatory graph of a change or an extension in the length of a two-dot line image due to the linear speed rate, when the two-dot line image (i.e., an image of two dots) that is formed on the latent image carrier in a direction orthogonal with the running direction is transferred onto the intermediate transfer medium (i.e., the intermediate transfer belt).
The abscissa represents a linear velocity ratio. When the linear speed rate is zero, that is, when the running speed of the latent image carrier is equal to that of the intermediate transfer medium, a value of 140 micrometers on the ordinate is the length of the two-dot line image on the latent image carrier, where one dot has 70 micrometers.
It is clear that when an absolute value of the linear speed rate in both the plus and minus sides increases, the length of the transferred two-dot line image increases, where a dot mark represents an actual measurement value, and straight lines 2-1 and 2-2 represent theoretical values.
The extension of the transfer toner image is determined based on a relative moving distance brought by the running speed difference. In other words, when the transfer toner image passes through the nip width of the transfer section at a constant speed difference Δv (=Vb−Va), the relative moving distance difference between the latent image carrier and the intermediate transfer medium becomes a product of a transmission time Tn and the speed difference Δv, that is, Tn times Δv.
The extension of the transfer toner image is not so conspicuous when the resolution of the image forming apparatus itself is low. However, under the recent situation that high resolution and a high-precision image are progressing, the extension of the transfer toner image becomes a serious problem.
The extension of the transfer toner image occurs due to the difference in the running speeds when the transfer toner image passes through the nip width of the transfer section. Therefore, in order to reduce the extension, the difference in the running speeds can be made smaller or the nip width can be made smaller. However, there is a physical limit to a reduction in the nip width. When the difference in the running speeds is made smaller, the effect of reducing the reverse transfer also decreases.